1. Field of the Invention
The present invention relates to a lubricating fluid composition for a dynamic pressure bearing which is used as the lubricating fluid of a dynamic pressure bearing which supports a shaft by dynamic pressure, and to the magnetic characteristics of which are utilized for sealing action.
2. Description of the Background
In recent years, attention has been directed to the bearing component of such devices as motors which drive various rotating members such as polygon mirrors, magnetic discs, optical discs, and the like at high speed. Attention has been directed to the use of a magnetic lubricating fluid as a lubricating fluid for the dynamic pressure bearing.
A magnetic fluid is a colloidal solution in which fine ferromagnetic particles are stably dispersed in a liquid solvent. In view of the strong ferromagnetic property which the liquid itself shows, the fluid is usually used as a sealant in combination with ball bearings in many cases. However, dynamic pressure bearings, which use the magnetic fluid as a lubricating fluid of the bearing, have great prospects, because the dynamic pressure bearing is superior in high speed rotation stability and stillness and the like in comparison to ball bearings. Dynamic pressure bearings provided with a magnetic fluid have been proposed. For example, Japanese Laid-Open Patent Application No. 88223/1985 discloses a device having a fluid which acts as both a lubricating fluid, for generating dynamic pressure, and a magnetic fluid for magnetic sealing.
A magnetic fluid composition has been used in the past for sealing purposes because of its most important property which is that of high sealing at pressures of about 10.sup.-6 Torr under high speed rotation conditions. However, a magnetic fluid composition which simultaneously satisfies both functions as a lubricating fluid and a sealing magnetic fluid (low abrasion characteristics and good lifetime characteristics of low volatility and high temperature resistance) has not yet been developed (see PETROTECH, vol. 13, No. 12 (1989)).
However, in such a device as mentioned above, the magnetic fluid composition must have both a low volatility as a seal and low viscosity to lower bearing loss. That is, the bearing portion of a device is exposed to far greater shearing forces and higher temperatures than the sealing portion, of a bearing device. Sometimes, active metal surfaces are formed by metal-metal contact and wear in the bearing, particularly caused at starting or stopping of a bearing containing device, with which the fluid comes into contact.
There is therefore a need for a lubricating fluid composition for dynamic pressure bearings which achieves not only high sealing properties, but also high lubricity and long lifetime characteristics.
As a result of research into the characteristics which a lubricating fluid composition should possess for use as both a lubricating fluid for dynamic pressure generation and a magnetic fluid for sealing, it has been found that it is necessary for the fluid to have:
(1) a low viscosity of 100 CP or less (at 27.degree. C.),
(2) a saturation magnetization of 50 gauss or more,
(3) low volatility which can maintain sealing characteristics,
(4) high temperature gelation resistance and oxidation resistance and the like and which exhibits stability in suppressing these effects, and
(5) restraint of the activity of metal caused by contact or wear. However, a magnetic fluid satisfying these characteristics has not yet been developed. Above all, as mentioned in (4) above, the phenomenon of so-called gelation, in which fluidity is lost, which is likely to be generated particularly at high temperatures is a serious problem which needs to be solved in order to improve the lifetime characteristics of a magnetic lubricant fluid.